1. Field of the Invention
The invention relates to device testing. More particularly, the invention relates to improved semiconductor device qualification testing.
2. Description of the Related Art
Semiconductor device fabrication is a complex process. During fabrication, semiconductor wafers are subjected to numerous complicated processes in order to impress upon them the desired characteristics. For example, a typical silicon wafer is subjected to extreme temperature and pressure variations during the deposition process. Although semiconductor fabrication has developed into a highly complex scientific craft, the process still retains an art-like nature in that each wafer which passes through the fabrication process tends to have different characteristics compared to the others. In fact, even within a single wafer, each die tends to develop different performance characteristics from its neighbors which were seemingly produced in an identical manner.
After a wafer of dies has been processed, the dies are separated and placed in device housings. The resulting integrated circuits retain the variations in performance characteristics inherent in the die from which they are created. Therefore, in a typical integrated circuit environment, integrated circuits are qualification tested in order to determine the device characteristics. The integrated circuits that have the best performance characteristics may be used in the most demanding environments. The integrated circuits with lower performance characteristics may be used in more cost-sensitive, less demanding applications.
The process of determining the performance characteristics of an integrated circuit is called qualification testing. For example, qualification testing is used to determine the performance of memory chips. A typical memory chip is comprised of millions of memory locations. Qualification testing can be used to determine whether each memory location on a newly fabricated memory chip is capable of accurately storing a bit of data. Even if less than all of the memory locations are functioning, the device is still useful in many applications. Therefore, a memory qualification process may determine the number of properly operating memory locations available on a memory device.
A device qualification process is typically simultaneously executed on a batch of integrated circuits. The performance characteristic categorization is referred to as a binout. Thus, fully functional devices binout as the highest performance devices. The reduced functionality devices binout in one or more categories of lower performance devices. Unfunctioning devices binout as rejected devices.
When a new qualification test is developed, it is critical that it provides proper device binout categorization before it is used for large scale production purposes. If the qualification test is mis-functioning, fully functional devices may be erroneously sold at a reduced price. Likewise, partially performing devices may be used in high performance applications in which the devices will fail.
The introduction of new qualification test software is inevitable. New qualification test software may be developed in order to perform more extensive tests or to increase the speed at which testing is performed. Before new qualification tests are used in the production environment, they themselves must be tested in order to assure that they are properly binning devices. However, determining the proper operation of a qualification test is not straight forward.
According to the prior art, two side by side test set ups are used to test a new qualification process. First, a functioning test process is performed in a first tester. The resulting binouts are stored. Then, the same devices are removed from the first tester and placed in the second tester. The devices are subjected to the trial testing process and the resulting binouts are stored. An attempt is made to determine whether the trial process is properly functioning by comparing the resulting binouts. For example, whenever a device is categorized differently by the known and the trial testing process, the device may be singled out as the subject of further testing to determine the cause of the variation.
Such testing has inherent disadvantages. Variations in the physical testers may cause variations in the binouts which are independent of the functioning of the trial testing process. In addition, the process of moving the devices from one tester to another may damage one or more of the devices causing a variation in the binouts which is independent of the functioning of the trial testing process. In addition, the process is time consuming and operator intensive.
Therefore, there has been a long felt need in the industry to have a means and method of providing reliable testing of qualification test software.